Restrict IMAP login, but allow Postfix SASL

Hi,

we have a very simple user-/passdb (like passwd) to authenticate virtual IMAP users. We also use this for Postfix authentication. Nothing special.

But, we need to exclude some of the users from IMAP login. This means, some users should be allowed to send mail via Postfix (submission) and therefore authenticate via SASL against dovecot successfully, but they should not be allowed to login to their IMAP mail box.

How could this be done?

Thanks! -lutzn